In the glass-making art it is known to form hollow articles with decorative surface patterns by cutting, laminating, etching or similar procedures.
Among these are the filigree techniques which were developed as early as the middle ages by Venetian glass masters and which were applied to transparent, colorless or colored glass for unique esthetic effects. These filigree techniques could be used in lieu of painting or engraving to generate a variey of patterns on glass articles of various shapes.
Improvements on the manually practiced Venetian filigree techniques led to a process by which a cylindrical mold of clay or iron, for example, is lined internally with uniformly spaced opaque, white or colored glass rods which are fixed to the inner wall of that mold, e.g. with clay, whereupon a hot, generally colorless parison of glass is inserted on a blow pipe into the mold and is expanded into contact with the rods which adhere to the parison and form a pattern thereon. The parison carrying these rods is reheated and stretched, twisted and otherwise deformed into a hollow conical body with an array of glass rods of reduced thickness extending helically around its circumference. Two such bodies with opposite twist could be intercalated and fused together to form a pattern of intersecting helicoidal lines.
With such a procedure, the production of more intricate, finely structured patterns is highly complicated.